To delve further into analyzing a room design it is necessary that we start defining the contents of the room. Some objects and materials are very reflective, like mirrors or ceramic tile. Others are broadband absorbers, like a cloth covered couch, and some are generally reflective but absorb well in a part of the band (1/2” drywall on 2x4 studs is strongly reflective at middle and high frequencies but absorbs fairly well around 125 Hz). What needs to be done is to take an inventory of all the major contents of the room as planned and count up the Sabin content. Sabins are a unit of measure which expresses the amount of sound energy, for a given range of frequencies, that a material (or in some cases an object) will absorb (vs. reflect). A Sabin number of 1.00 expresses the concept that a square foot of this material hung on the wall for example, is like a one square foot open window – it will reflect no sound energy, all will be absorbed [or pass through the window and leave the room]. A Sabin number of 0.00 implies that a material will reflect all of the sound energy that strikes it. Sabins are however a simple way of looking at something very complicated, and are not completely accurate – for example the absorption that takes place will depend on the angle that sound wave strikes a sample of a material – take Sabin numbers as a general guide only, not a fixed and perfectly accurate description of the acoustic effect introducing a given amount of material into a room. The nature of the room, and the size, shape, and placement of the absorptive material, and the direction of the sound impinging on the material all can either increase or reduce the abortiveness of a material.

Predicting modes in non-rectangular spaces is very complicated and requires specialized software. Knowing a room’s volume and its Sabin content it is possible to estimate many of a room's basic acoustic properties including the strength of its reverberant field. This estimate is generally expressed as a room’s RT60 which is short hand for the time in seconds that a sound will take to decay to 60 dB below its initial impulse energy level. A high RT60 means that sound will bounce around for a long time before it is absorbed. Dead rooms have low RT60’s. Keep in mind that only large rooms can have a "real" RT60 - medium and small rooms can't support a random field of reverberations - they are just to small. Nonetheless, these calculations can help one estimate how much absorptive treatment will be needed to tame the "boom" in any given proposed room.

Estimates of this sort are helpful in the design process for many purposes, but the main purpose you should concern yourself with is that of estimating how much absorptive treatment a room will need. Smaller rooms cannot tolerate high RT60’s (long reverb times) without introducing strong colorations to the low frequency part of the reverberant field. Even large rooms will need to limit reverb times in order to be considered good sounding rooms for various purposes. Generally, for a tracking/recording room, the lower the low frequency energy expected, and the larger the room, the higher the RT60 can/should be.

A 5,000 cubic foot recording room for modern rhythmic music (with oodles of low frequency content) might need to be limited to an average RT60 of .6 seconds, though significantly longer RT60’s for the lower end of the spectrum are common and generally considered acceptable (up to an 80% rise over the 1 kHz RT60 for sounds in the 63 Hz band are not considered objectionable by many), so a recording room with a .6 second RT60 at 1 kHz and a 1.1 second RT60 at 60 Hz would be a workable space according to many. This concept tracks the reverberations times one might find in a typical living room, where the RT60 for lower frequencies would be also be significantly greater than that for mid and high frequencies.

Here is an Excel spreadsheet that can help you estimate the modal behavior and proper target reverb time or a room of any size, for various purposes.